: The main hypothesis of the proposed doctoral studies is that traffic and residential wood burning are important contributors of particulate matter (PM) and elemental carbon (EC) to indoor environments, where people spend most of their time. Particulate matter is of importance because epidemiological studies have identified a correlation between daily ambient particle concentrations and increased morbidity and mortality. Elemental carbon is one of the components of PM. Elemental carbon will be used as a tracer for diesel traffic and wood burning because the only known source of aerosol EC in the atmosphere is the combustion of carbonaceous fuels. In addition of EC wood burning and diesel engines generate products of incomplete combustion, including carbon monoxide, and organic matter such as polycyclic aromatic hydrocarbons (PAHs). Studies have shown that extractable organic matter (which contains PAHs) from particles generated by traffic and residential wood burning are mutagenic and carcinogenic. Instruments which measure PM and EC concentrations in real time will be used to distinguish between indoor and outdoor sources. The study will involve simultaneous indoor and outdoor sampling of PM and EC using direct-reading instruments. Sampling will be conducted for a year at two locations: an urban community heavily impacted by vehicular traffic, and a suburban community with less vehicular traffic, but where wood burning occurs regularly in the fall and winter months. Results from this study will more fully establish the causal association between PM sources and exposure in the urban environment so that public health can be more fully assessed.